There are two important aspects while developing the future flat display, one is how to manufacture a flexible, light, and thin display panel, and the other is how to manufacture electronic elements with higher electrons mobility and higher response speed for display panel. But the conventional flat displays are using the glass material as base substrates, which is superior in large area manufacturing and mass production; however, the feature of light, thin and flexible is difficult to be put into practice for glass substrate, therefore, undoubtedly, there is no better way to overcome the drawback above unless find an appropriate material. As to the other aspect, Low Temperature PolySilicon, i.e. LTPS technology, can achieve the objective; therefore, the thin film transistors formed by process of LTPS gradually becomes the main stream to be substituted for the process forming amorphous thin film transistors.
Among the materials nominated for the purpose to make a flexible, light, and thin display, plastic substrate is substantially win the engineers' gaze, nevertheless, the plastic substrate can't withstand the damage caused by heat generated during the manufacturing process in LTPS manufacturing process. This is because, during such process, a laser annealing with processing temperature more than 600-Celsius degree, which is almost higher than the glass transition temperature of plastic substrate, is necessary to be utilized to transform the amorphous silicon into poly-crystalline silicon.
Although plastic substrates are not capable of bearing such high temperature, overall speaking, compared with other materials, the plastic substrates still have many merits that engineers can't give up; therefore, there are still many efforts that scientists and engineers dedicate to overcome such as U.S. Pat. No. 5,817,550 and U.S. Pat. No. 6,680,485. In the U.S. Pat. No. 5,817,550, it disclosed a method utilizing a low energy laser, which is to form poly-crystalline silicon on a plastic substrate. In such method, at first, a silicon dioxide is formed on a plastic substrate, and then an amorphous layer was deposited on said silicon dioxide layer. Subsequently, a short-pulse XeCl Excimer Laser (308 nm) is used to transform said amorphous silicon layer into poly-crystalline silicon in no more than 100 ns. Another U.S. Pat. No. 6,680,485 discloses a method utilizing a low energy laser to form poly-crystalline silicon on a low-temperature plastic substrate, wherein a specific thickness around 0.1 to 5.0 micrometer of silicon dioxide is formed, and then a specific thickness around 10 to 500 nanometer is formed on said silicon dioxide layer. Subsequently, a short-pulse XeCl Excimer Laser (308 nm) is used to transform said amorphous silicon layer into poly-crystalline silicon with processing temperature no more than 250-Celsius degree.
Summarizing the prior arts described above, a list of drawbacks was concluded as following:                (1) The grain size of poly-crystalline silicon will be affected by lower the energy of laser and shorten the annealing time, and then characteristics of elements formed on plastic substrate will be further influenced.        (2) The prior arts disclose a step to form a silicon dioxide on the plastic substrate; however, forming silicon dioxide is time-consuming so that throughput of mass production will be affected. For example, it will spend 30 minutes to 40 minutes forming 4-micrometer silicon dioxide layer.        (3) Meanwhile, although the better isolation can be achieved by thicker oxidized layer, the brittle characteristic of the thicker oxidized layer can also leads to easy chapping and easy fragile that result in difficulty to control the manufacturing process.Hence, it is necessary to develop a structure of thermal resistive layer for plastic substrate and manufacturing method thereof to overcome the drawbacks of the prior arts.        